1. Field of the Invention
This invention relates to degradable polyacetal polymers and therapeutic agents derived therefrom, the production of these materials, and methods of disease treatment using them.
2. Description of the Related Art
Polymer therapeutics (R. Duncan, “Polymer therapeutics for tumor specific delivery”, Chem. & Ind., 7, 262–264, 1997) are developed for biomedical applications requiring physiologically soluble polymers; and include biologically active polymers, polymer-drug conjugates, polymer-protein conjugates, and other covalent constructs of bioactive molecules. An exemplary class of a polymer-drug conjugate is derived from copolymers of hydroxypropyl methacrylamide (HPMA), which have been extensively studied for the conjugation of cytotoxic drugs for cancer chemotherapy (R. Duncan, “Drug-polymer conjugates: potential for improved chemotherapy”, Anti-Cancer Drugs, 3, 175–210, 1992; D. Putnam et al., “Polymer conjugates with anticancer activity”, Adv. Polym. Sci., 122, 55–123, 1995; R. Duncan et al., “The role of polymer conjugates in the diagnosis and treatment of cancer”, STP Pharma, 6, 237–263, 1996). An HPMA copolymer conjugated to doxorubicin, known as PK-1, is currently in Phase II evaluation in the UK. PK-1 displayed reduced toxicity compared to free doxorubicin in the Phase I studies (P. Vasey et al., “Phase I clinical and pharmacokinetic study of PKI (HPMA copolymer doxorubicin): first member of a new class of chemotherapeutic agents: drug-polymer conjugates” Clin. Cancer Res., 5, 83–94. 1999). The maximum tolerated dose of PK-1 was 320 mg/m2, which is 4–5 times higher than the usual clinical dose of free doxorubicin.
The polymers used to develop polymer therapeutics may also be separately developed for other biomedical applications that require the polymer be used as a material. Thus, drug release matrices (including microparticles and nanoparticles), hydrogels (including injectable gels and viscous solutions) and hybrid systems (e.g. liposomes with conjugated poly(ethylene glycol) on the outer surface) and devices (including rods, pellets, capsules, films, gels) can be fabricated for tissue or site specific drug delivery. Polymers are also clinically widely used as excipients in drug formulation. Within these three broad application areas: (1) physiologically soluble molecules, (2) materials, and (3) excipients, biomedical polymers provide a broad technology platform for optimizing the efficacy of an active therapeutic drug.
An increasing number of physiologically soluble polymers have been used as macromolecular partners for the conjugation of bioactive molecules. Many polymers have the disadvantage of being non-degradable in the polymer backbone. For example, poly(ethylene glycol) (C. Monfardini et al., “Stabilization of substances in circulation”, Bioconjugate Chem., 9, 418–450, 1998; S. Zalipsky, “Chemistry of polyethylene glycol conjugates with biologically active molecules”, Adv. Drug Delivery Rev, 16, 157–182, 1995; C. Delgado et al., “The uses and properties of PEG-liked proteins”, Crit. Rev. Ther. Drug Carrier Syst., B, 249–304, 1992; M. L. Nucci et al., “The therapeutic values of poly(ethylene glycol)-modified proteins”, Adv. Drug Delivery Rev. 6, 133–151, 1991; A. Nathan et al., Copolymers of lysine and polyethylene glycol: A new family of functionalized drug carriers”, Bioconjugate Chem. 4, 54–62, 1993) and HPMA (D. Putnam et al., “Polymer conjugates with anticancer activity”, Adv. Polym. Sci., 122, 55–123, 1995; and R. Duncan et al., “The role of polymer conjugates in the diagnosis and treatment of cancer”, STP Pharma, 6, 237–263, 1996) copolymers have been extensively studied for conjugation. PEG is also generally used in the pharmaceutical industry as a formulation excipient. These hydrophilic polymers are soluble in physiological media, but their main disadvantage is that the polymer mainchain does not degrade in vivo. Thus it is not possible to prohibit accumulation of these polymers in the body. Only polymers with a molecular weight lower than the renal threshold can be used for systemic administration. It is imperative that for the systemic use of non-degradable polymers such as BPMA and PEG only molecules of a molecular weight which are readily cleared be administered or else long-term deleterious accumulation in healthy tissue will invariably result (L. Seymour et al., “Effect of molecular weight (Mw) of N-(2-hydroxypropyl)methacrylamide copolymers on body distributions and rate of excretion after subcutaneous, intraperitoneal and intravenous administration to rats”, J. Biomed. Mater. Res. 21, 341–1358, 1987; P. Schneider et al., “A review of drug-induced lysosomal disorders of the liver in man and laboratory animals”, Microscopy Res. Tech. 36, 253–275, 1997; C. Hall et al., “Experimental hypertension elicited by injections of methyl cellulose”, Experientia 17, 544–454, 1961; C. Hall et al., “Macromolecular hypertension: hypertensive cardiovascular disease from subcutaneously administered polyvinyl alcohol”, Experientia 18, 38–40, 1962).
Although some natural polymers such as polysaccharides have the advantage of being degradable in vivo, e.g. dextran, they typically lack a strict structural uniformity and have the propensity upon chemical modification (i.e. conjugation of a bioactive molecule) to become immunogenic or non-degradable (J. Vercauteren et al., “Effect of the chemical modification of dextran on the degradation by dextranases”, J. Bio. Comp. Polymers 5, 4–15, 1990; W. Shalaby et al., “Chemical modification of proteins and polysaccharides and its effect on enzyme-catalyzed degradation”, in: S. Shalaby, ed. Biomedical Polymers. Designed-to-degrade systems. New York: Hanser Publishers, 1994). Other polysaccharides which have been investigated for biomedical conjugation applications include chitosan (Y. Ohya et al., “α-1,4-Polygalactosamine immobilised 5-fluorouracils through hexamethylene spacer groups via urea bonds”, J. Cont. Rel., 17, 259–266, 1991), alginate (A. Al-Shamkhani et al., “Synthesis, controlled release properties and antitumor activity of alginate cis-aconityl daunomycin conjugates”, Int. J. Pharm., 122, 107–119, 1995; S. Morgan et al., “Alginates as drug carriers: covalent attachment of alginates to therapeutic agents containing primary amine groups”, Int. J. Pharm., 122, 121–128, 1995), hyaluronic acid (B. Schechter et al., “Soluble polymers as carriers of cisplatinum”, J. Cont. Rel., 10, 75–87, 1989), 6-O-carboxymethyl chitan (Y. Ohya et al., “In vivo and in vitro antitumor activity of CM-Chitin immobilized doxorubicins by lysosomal digestible tetrapeptide spacer groups”, J. Bioact. Compat. Polymers, 10, 223–234, 1995) and 6-O-carboxymethyl pullulan (H. Nogusa et al., “Synthesis of carboxymethylpullulan peptide doxorubicin conjugates and their properties”, Chem. Pharm. Bull., 43, 1931–1936, 1995).
Other natural polymers such as proteins can also be used to conjugate a bioactive molecule. For example albumin has been investigated as a protein used to conjugate a bioactive molecule (P. Balboni et al., “Activity of albumin conjugates of 5-fluorodeoxyuridine and cytosine arabinoside on poxyiruses as a lysosomotropic antiviral chemotherapy”, Nature, 264, 181–183, 1976; A. Trouet et al., “A covalent linkage between daunorubicin and proteins that is stable in serum and reversible by lysosomal hydrolases as required for a lysosomotropic drug-carrier conjugate. In vitro and in vivo studies”, Proc. Natl. Acad. Sci. USA, 79, 626–629, 1982; F. Dosio et al., “Preparation, characterization and properties in vitro and in vivo of a paclitaxel-albumin conjugate”, J. Cont. Rel., 47(3), 293–304, 1997; T. Yasuzawa et al, “Structural determination of the conjugate of human serum albumin with a mitomycin C derivative, KW-2149, by matrix assisted laser desorption/ionization mass spectrometry”, Bioconjugate Chem., 8, 391–399, 1997; A. Wunder et al., “Antitumor activity of methotrexate-albumin conjugates in rats bearing a Walker-256 carcinoma”, Int. J. Cancer, 76, 884–890, 1998). The major limitations for using a protein to conjugate a bioactive compound include the propensity for inducing immunogenicity and non-specific degradation of the protein in vivo, and denaturation and irreversible alteration of the protein during preparation of the conjugate. Other proteins such as transferrin, which binds to the transferrin receptor and thus have the potential to undergo receptor-mediated uptake (T. Tanaka et al., “Intracellular disposition and cytotoxicity of transferrin-mitomycin C conjugate in HL60 cells as a receptor-mediated drug targeting system”, Biol. Pharm. Bull, 21(2), 147–152, 1998) and various immuno-conjugates (D. Gaal et al., “Low toxicity and high antitumor activity of daunomycin by conjugation to an immunopotential amphoteric branched polypeptide”, Eur. J. Cancer, 34(1), 155–16, 1998; P. Trail et al., “Site-directed delivery of anthracyclines for the treatment of cancer”, Drug Dev. Res. 34, 196–209, 1995; E. Eno-Amooquaye et al., “Altered biodistribution of an antibody—enzyme conjugate modified with polyethylene glycol”, Br. J. Cancer, 73, 1323–1327, 1996; P. Flanagan et al., “Evaluation of antibody-[N-(2-hydroxypropyl)methacrylamide] copolymer conjugates as targetable drug-carriers. 2. Body distribution of anti Thy-1,2 antibody, anti-transferrin receptor antibody B3/25 and transferrin conjugates in DBA2 mice and activity of conjugates containing daunomycin against L1210 leukemia in vivo”, J. Cont. Rel., 18, 25–38, 1992; C. Springer et al., “Ablation of human choriocarcinoma xenografts in nude mice by antibody-directed enzyme prodrug therapy (ADEPT) with three novel compounds”, Eur. J. Cancer, 11, 1362–1366, 1991.) also have been investigated. Monodisperse molecular weight distribution is often claimed to be a significant advantage for using proteins to conjugate drugs, but this can only be useful if a single species of the protein-drug conjugate can be reproducibly prepared on adequate scale which stable on storage. This is generally not economically or technologically possible to achieve in practice. Thus, there is a need for degradable synthetic polymers developed for biomedical application, and specifically for conjugation applications, which can address the limitations inherent in the use of natural polymers for these applications.
Synthetic polymers which have been prepared and studied that are potentially degradable include polymers derived from amino acids (e.g. poly(glutamic acid), poly[5N-(2-hydroxyethyl)-L-glutamine), β-poly(2-hydroxyethyl aspartamide), poly(L-glutamic acid) and polylysine). These polymers when prepared for conjugation applications that require physiological solubility do not degrade in vivo to any extent within a time period of 10–100 hours. Additionally polymers and copolymers including pseudo-poly(amino acids) (K. James et al., “Pseudo-poly(amino acid)s: Examples for synthetic materials derived from natural metabolites”, in: K. Park, ed., Controlled Drug Delivery: Challenges and Strategies, Washington, D.C.: American Chemical Society, 389–403, 1997) and polyesters such as copolymers of polylactic and poly(glycolic acid), poly(a or b-malic acid) (K. Abdellaoui et al., “Metabolite-derived artificial polymers designed for drug targeting, cell penetration and bioresorption”, Eur. J. Pharm. Sci., 6, 61–73, 1998; T. Ouchi et al., “Synthesis and antitumor activity of conjugates of poly (a-malic acid) and 5-fluorouracil bound via ester, amide or carbamoyl bonds”, J. Cont. Rel, 12, 143–153, 1990), and block copolymers such as PEG-lysine (A. Nathan et al., “Copolymers of lysine and polyethylene glycol: A new family of functionalized drug carriers”, Bioconjugate Chem., 4, 54–62, 1993.), poly(lysine citramide) (K. Abdellaoui et al, “Metabolite-derived artificial polymers designed for drug targeting, cell penetration and bioresorption”, Eur. J. Pharm. Sci., 6, 61–73, 1998) and amino acid-PEG derived block copolymers (G. Kwon et al., Block copolymer micelles as long-circulating drug vehicles”, Adv. Drug Del. Rev., 16, 295–309, 1995; and V. Alakhov et al., “Block copolymeric biotransport carriers as versatile vehicles for drug delivery”, Exp. Opin. Invest. Drugs, 7(9), 1453–1473, 1998) have also been investigated for conjugation.
Acetals are well known to be hydrolytically labile under mildly acidic conditions. Thus, biomedical polymers possessing acetal linkages in the polymer mainchain may undergo enhanced rates of hydrolysis in biological environments that are mildly acidic compared to biological environments that are at neutral or basic pH. For example, soluble polyacetals that can conjugate a bioactive molecule are expected to degrade at enhanced rates at the acetal functionality during cellular uptake because of the increase in acidity during endocytosis. Polyacetals will also display enhanced rates of hydrolysis in acidic regions of the gastrointestinal tract. Additionally polyacetals would be expected to degrade at enhanced rates at sites of diseased tissue that are mildly acidic (e.g. solid tumors).
Preparing polyacetals can be accomplished by acetal- or transacetalization reactions which result in the formation of a low molecular weight by-product (e.g. water or an alcohol). Complete removal of such a by-product is necessary for reproducible polymerization and to ensure the polyacetal does not degrade on storage. Usually harsh conditions are required to obtain high molecular weight polymer. If functionalized monomers relevant for biomedical applications are used, such conditions can often lead to unspecified chemical changes in the monomer. Polyacetals can be prepared without generation of a small molecule which requires removal by cationic ring-opening polymerization using bicyclic acetals (L. Torres et al., “A new polymerization system for bicyclic acetals: Toward the controlled/“living” cationic ring-opening polymerization of 6,8-dioxabicyclo[3.2.1] octane”, Macromolecules, 32, 6958–6962, 1999). These reaction conditions lack versatility because they require bicyclic acetal monomers that are difficult to prepare with a wide range of chemical functionality useful for conjugation applications.
Polyacetals can also be prepared without generation of a small molecule byproduct that requires removal by the reaction of diols and di-vinyl ethers using an acid catalyst, as described by Heller (J. Heller et al., “Preparation of polyacetals by the reaction of divinyl ethers and polyols”, J. Polym. Sci.: Polym. Lett. Ed., 18, 293–297, 1980; J. Heller et al., “Polyacetal hydrogels formed from divinyl ethers and polyols”, U.S. Pat. No. 4,713,441, 1987). Such polyacetals have uniform structure in that they are strictly alternating polymers of the A-B type. Uniform structure in biomedical polymer development is critical for optimization of the biological profile and to ensure the polymer meet regulatory requirements. The polymerization of diols and di-vinyl ethers occurs without the elimination of a small molecule under mild conditions. This is more efficient than polymerizations where there is a molecule (e.g. water or methanol) which must be removed. Polyacetals suitable for conjugation can be prepared by utilization of suitably functionalized diols, di-vinyl ethers, and/or hydroxy-vinyl ether monomers. Thus it becomes possible to prepare polyacetals for conjugation that possess conjugation functionality on either the A or B monomeric unit. Such structural uniformity is advantageous for controlling conjugation of bioactive molecules along the polymer mainchain.
The production of biodegradable polyacetals derived from polysaccharides which chemically has been described in WO 96/32419. This approach does not give polymeric materials displaying structural uniformity and suffers from the aforementioned limitations where chemical modification (i.e. conjugation of a bioactive molecule) often leads to the polysaccharide to become immunogenic or non-degradable. It is not possible to prepare polymeric materials displaying an alternating A-B structure, rather the structure of these polysaccharide derived polyacetals are too diverse to chemically analyze to the degree necessary to fulfill regulatory requirements.
The disclosures of these and other documents referred to throughout this application are incorporated herein by reference in their entirety.